(a) Field of the Invention
The present invention relates to a three-dimensional electrode for electrolysis having elastic electroconductive sections, an electrolytic cell employing the three-dimensional electrode, and a method of electrolysis using the three-dimensional electrode.
(b) Description of the Related Art
Electrolysis industry including chloroalkali electrolysis has an important-role-in-material industry as its typical industry. In addition to this important role, energy-saving is earnestly required in a country where energy cost is high such as in Japan because the energy consumed in the chloroalkali electrolysis is higher.
The chloroalkali electrolysis has been converted from the mercury method into the ion exchange membrane method through the diaphragm method in order to solve the environmental problems and to achieve the energy-saving, and actually the energy-saving by about 40% has been attained in about 25 years. However, even the energy-saving to this extent is unsatisfactory, and as far as the present method is used, the further electric power saving is impossible while the cost of the energy or the electric power occupies about half of the total manufacture cost.
In the electrolytic cell mounting a hydrogen-generating cathode used for brine electrolysis, cell voltage is reduced by disposing an anode, an ion exchange membrane and the hydrogen-generating cathode in intimate contact with one another. However, in a large-scaled electrolytic cell with an electrolytic area reaching several square meters where an anode and a cathode are made of rigid materials, an inter-electrode distance can be hardly maintained at a specified value by intimately contacting both electrodes on an ion exchange membrane.
In order to reduce the inter-electrode distance or a distance between the electrode and the corresponding electrode current collector or to maintain these at a nearly fixed value, an electrolytic cell using an elastic material therein is proposed.
The elastic material includes a non-rigid material such as a woven fabric, a non-woven fabric and a mesh, and a rigid material such as a blade spring.
The use of the non-rigid material arises such problems that the inter-electrode distance becomes non-uniform due to the partial deformation of the non-rigid material generated by the undue pressing from the counter-electrode side and the fine wires of the non-rigid material stick to an ion exchange membrane. The rigid material such as the blade spring inconveniently damages the ion exchange membrane, and reuse thereof may become impossible due to plastic deformation.
Various methods have been proposed for pressing the electrodes toward the ion exchange membrane in the ion exchange membrane electrolytic cell such as an electrolytic cell for brine electrolysis because the lower-voltage operation is desirable by intimately contacting the anode and the cathode with the ion exchange membrane.
As described, the structural characteristic of the electrolytic cell sandwiching the ion exchange membrane between the anode and the cathode is that, in order to prevent the damage of the ion exchange membrane by means of the uniform contact between the electrode and the ion exchange membrane and to maintain the inter-electrode distance to be minimum, at least one of the electrodes can freely move in the direction of the inter-electrode distance so that the electrode is pressed by an elastic element to adjust a holding pressure.
The elastic element includes a knitted fabric and a woven fabric made of metal wires or a structure prepared by stacking the fabrics, or by three-dimensionally knitting the fabrics or by three-dimensionally knitting the fabrics followed by crimp processing, and a non-woven fabric made of metal fibers, a coil hopper (spring) and a blade spring. These examples have spring elasticity of some kind.
On the other hand, the blade spring and the metal mesh are used for smoothly conducting the power supply from the current collector to the electrode in an industrial electrolytic cell such as that for brine electrolysis.
As described, however, the blade spring and the metal mesh are so rigid as to damage the ion exchange membrane and may provide the insufficient electric connection due to its lower deformation rate.
In order to solve these problems, an electrolytic cell is disclosed in JP-B-63(1988)-53272 (FIGS. 1 to 8) in which a cathode is uniformly pressed toward a diaphragm to intimately contact the respective elements with one another by mounting a metal coil in place of the metal mesh between the cathode and the cathode end wall.
The extremely small diameter and the higher deformation rate of the metal coil sufficiently contact the respective elements with one another so that the stable operation of the electrolytic cell is possible.
However, in the electrolytic cell disclosed in the above JP-B-63(1988)-53272, the metal coil in addition to the anode and the cathode is mounted in the electrolytic cell so that the number of the elements increases and the cathode, if rigid, cannot provide the sufficient adhesion.
In order to solve the defects, an electrode consisting of a metal coil which supports electrode catalyst or another electrode formed by winding the metal coil around a anti-resistant frame has been proposed (JP-A-2004-300543). This technique is characterized by using the metal coil as the electrode itself and not by using the metal coil for pressing the electrode toward the ion exchange membrane. This electrode has an advantage that caustic soda can be produced with a higher efficiency because the higher strength and the higher toughness of the electrode retain its shape for a longer period of time so that the ion exchange membrane is neither mechanically damaged nor excessively deformed to result in the insufficient power supply. In spite of the above-described advantages, this electrode has a disadvantage of requiring a lot of manufacturing labor.
In the meantime, for the effective utilization of lumber resources, high yield production of chemical pulp is important. A polysulfide cooking process is proposed as a tool of high yield production of kraft pulp which is a mainstream of the chemical pulp. The cooking liquor in the polysulfide cooking process is prepared by oxidizing alkali aqueous solution containing sodium sulfide or white liquor with molecular oxygen such as air under presence of catalyst such as active carbon.
In this method, the polysulfide cooking liquor having polysulfide concentration of about 5 g/liter can be obtained at an inversion rate of about 60% and a selection rate of about 60% based on the sulfide ion. However, in this method, thiosulfate ion which does not at all contribute to the cooking is collaterally produced so that the cooling liquor containing the higher concentration polysulfide ion is hardly prepared at the higher selection rate.
The polysulfide ion herein also referred to as “polysulfide sulfur” includes, for example, sulfur having a valence “0” in sodium polysulfide (Na2Sx), that is, (x-1) atoms of the sulfur.
On the other hand, WO95/00701 discloses a method of electrolytically preparing polysulfide cooking liquor. In this method, an anode is used which is fabricated by coating a substrate with an oxide of ruthenium, iridium, platinum or palladium. Specifically, a three-dimensional mesh electrode having a substrate prepared by combining a plenty of expanded metals is disclosed.
JP-A-2000-515106 also discloses a method of electrolytically preparing polysulfide cooking liquor in which a porous anode made of carbon, especially accumulated carbon fibers having a diameter of 1 to 300 μm is used.
When starting electrolyte contains impurities, the above electrode used for the white liquor electrolysis (electrolytic preparation of polysulfide cooking liquor) or used for the other electrolysis, the impurities adhere to the electrode surface to increase the cell voltage. In order to avert this problem, the electrode is required to be washed, and at worst periodically replaced.
The impurities deposited on the interior of porous material are not sufficiently removed by physical washing, and chemical washing using acid or chelate is required for removing the impurities so that equipment expenses increase and the handling thereof is burdensome.
When the electrolyte containing the impurities is electrolyzed by using the conventional electrode, the impurities are deposited on the electrode surface and exert adverse influence to the membrane so that an operation for a longer period of time is hindered.